feat: Reimplement richtext using Bluesky-inspired facet system

- Fixed typo "filesred" → "files" in validate-lexicon/main.go
- Added validate-lexicon binary to .gitignore and removed from git
- Updated facet schema to include required $type fields for AT Protocol compatibility
- Removed duplicate mentions field from microblog.json (use facets instead)
- Added comprehensive facet tests covering UTF-8 byte counting and all feature types
- Fixed lexicon validation test to reference correct schema names
- Added detailed facet documentation with UTF-8 byte counting examples

The facet implementation now follows AT Protocol standards with proper $type
fields for each feature (mention, link, bold, italic, strikethrough, spoiler).
All byte indices use UTF-8 encoding for cross-platform consistency.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

- Replace separate markup/mention/link lexicons with unified facet approach
- Use byte-indexed positioning (byteStart/byteEnd) for text annotations
- Support overlapping features: bold, italic, strikethrough, spoiler, mention, link
- Add native support for community mentions with \! prefix
- Update all schemas to use *Facets fields instead of *Markup
- Align with AT Protocol standards for better federation compatibility

BREAKING CHANGE: All richtext markup fields renamed to facets

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Implement query and procedure lexicon definitions